Disperse dyes and pigments have hitherto been used as ink colorants. When disperse dyes and pigments are used in inks, the disperse dye or pigment must be dispersed in water in a finely divided form. But, disperse dyes and pigments are insoluble in water, and so a dispersant is important for rendering the disperse dye or pigment into a dispersed state and stably maintaining that dispersed state for a prolonged period of time. Nonionic surfactants, anionic surfactants and polymer surfactants have been proposed as such dispersants.
For example, JP-A 2000-290578 and JP-A 2002-020673 disclose the use of acetylene group-containing nonionic surfactants as pigment dispersants. However, although acetylene group-containing nonionic surfactants are excellent for the penetrating ability and foam-inhibiting ability of inks, they tend to have a poor dispersion performance; for example, dispersion takes a longer time than with other dispersants.
JP-A 2010-111826 discloses a pigment dispersion containing two or more types of dispersants selected from among sodium naphthalene sulfonate formalin condensates, polycarboxylic acid graft polymers and polyoxyethylene styrylphenyl ethers. Yet, given the high amount of dispersant addition relative to the pigment, there is room here too for improvement. Moreover, because foaming readily arises with the use of these dispersants, wetting agents and penetrants must be added for use in inkjet inks and the like.
The following are used as dispersants for disperse dyes and pigments:
styrene-acrylic acid-alkyl acrylate copolymers (in WO 2014/156758), formalin condensates of naphthalenesulfonic acids (in JP-A H08-127981), and styrene-acrylic copolymers (in JP-A 2016-175995). However, when these polymer compounds are used as the dispersant, the high viscosity of the dispersion makes preparation difficult. Also, in cases where the dispersion has been rendered into an ink composition, there is also the drawback that the composition dries easily. In addition, JP-A 2011-174007 uses nonionic surfactants such as polyoxyalkylene alkyl ethers, but these have dispersion performances that fall short of what is desired.